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Properties of Cytosinium Hydrogen Selenite

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Growth, Optical, Thermal and Mechanical Properties of Cytosinium hydrogen selenite: A Novel Nonlinear Optical Single Crystal

  • P. Jaikumar, S. Sathiskumar, T. Balakrishnan and K. Ramamurthi

 

Abstract

A novel nonlinear optical single crystal of cytosinium hydrogen selenite was successfully grown from aqueous solution by slow evaporation method at room temperature. The unit cell parameters of the grown crystal were estimated by single crystal X- ray diffraction analysis. The grown crystals were characterized by powder X- ray technique. Presence of various functional groups was identified from Fourier transform infrared spectroscopy. The optical transmittance and absorbance spectra recorded for the grown crystal show that the crystal possesses good transparency in the entire visible region. The dielectric constant and dielectric loss were calculated for the grown crystal as a function of frequency at different temperatures. Etching study of the grown cytosinium hydrogen selenite crystal was carried out with different etching time.

Keywords: Crystal growth; Optical properties; Mechanical properties; Dielectric constant, second harmonic generation efficiency

*Corresponding author Tel.: +91 - 9443445535

E - Mail addresses: [email protected] (T. Balakrishnan).

1.Introduction

In the past few decades, a myriad of efforts has been made in the design and characterization of nonlinear optical (NLO) materials due to their excellent properties and important applications in the field of laser technology, telecommunication, optoelectronic and photonic applications [1-2]. A large number of various organic and semiorganic NLO materials were synthesized and characterized. Cytosine is one of the pyrimidine derivatives consists of heterocyclic compound along with aromatic amine and keto groups [3-4]. Cytosine plays an important role in DNA/RNA base pairing, through several hydrogen-bonding pattern, and controls the essential features of life as it is involved in genetic codon of 17 amino acids [5]. The single crystal structure of anhydrous cytosine [6] and cytosine monohydrate [7] was reported. Tu Lee and and Pu Yun Wang [8] reported the molecular recognition of an achiral cytosine with oxalic acid, malonic acid and succinic acid. Babulal Das and Baruah [9] reported the hydrogen bonded single crystals of cytosine with adipic acid and citric acid. Reji Thomas and Kulkarni [10] studied the hydrogen bonding in proton transfer complexes of cytosine with trimesic acid and pyromellitic acid. Single crystal structure of metal complexes of cytosine with cobalt chloride [11], copper chloride [12] and calcium chloride [13] were reported. The single crystal structure of cytosinium hydrogen selenite (CHS) was reported by Radhwane Takouachet et al. [14]. A survey of literature shows no systematic works available on the growth of cytosinium hydrogen selenite single crystal and its characterization. Hence in this work we report on the synthesis and growth of CHS single crystal and characterization of the grown crystal for its structural, optical, nonlinear optical, dielectric, thermal and etching properties for the first time.

2. Experimental details

2.1 Synthesis

Aqua solution of CHS was prepared from equimolar amounts of AR grade cytosine and selenous acid (E - Merck). The reactants were thoroughly dissolved in doubly distilled water and stirred well for about three hours using temperature controlled magnetic stirrer to obtain a homogeneous mixture of solution. Evaporation of the prepared solution at room temperature yielded the product of CHS. Successive re-crystallization process was adapted to improve the purity of the synthesized CHS.

2.2 Crystal Growth

Saturated solution of CHS was prepared at room temperature using recrystallized salt in double distilled water and filtered using Whatman filter paper. The filtered solution was taken in a fresh beaker closed with perforated polythene sheet and kept in a dust free atmosphere for crystallization. Slow evaporation method yielded single crystals of size 4 × 2 × 2 mm3 and were harvested in a period of 15 days. The grown CHS crystals are shown in Fig.1.

Fig.1. As grown CHS crystals

3. Results and Discussion

3.1 X-ray diffraction studies

The grown single crystal was subjected to single crystal X-ray diffraction analysis at room temperature using Enraf Nonius CAD4 X – ray diffractometer with Mo Kα (λ = 0.7107Å) radiation to estimate the unit cell parameters. Single crystal structure studies show that CHS crystal belongs to orthorhombic system with a non - centrosymmetric space group PCa21. The unit cell parameters obtained are a = 7.024 Å (7.005 Å), b = 8.661 Å (8.634 Å), c = 12.741 Å (12.713 Å) and V= 771 Å3 (768 Å 3 ) and these values agree well with the corresponding values reported by Radhwane Takouachet et al. [14] given in parenthesis.

Powder X-ray diffraction pattern of the CHS crystal was recorded on Reich Seifert diffractometer using Cu Kα (λ = 1.5418 Å) radiation. The powdered sample was scanned over a 2θ range 10° - 80° at a scan rate of 1°/min. The recorded powder X - ray diffraction peaks were indexed using AUTOX 93 software. The indexed powder X - ray diffraction peaks of CHS are shown in Fig.2.

Fig.2. Powder X-ray diffraction pattern of CHS

3.2 Fourier Transform Infrared Spectral analysis.

The Fourier Transform Infrared spectral analysis of CHS crystal was carried out in the range of 400 – 4000 cm-1 using Perkin Elmer FT – IR spectrometer by the KBr pellet method to study the presence of various functional groups. The recorded FT-IR spectrum is shown in Fig. 3. In the higher energy region, the peak appears at 3316 cm-1 is assigned to NH2 asymmetric stretching vibration. The peak at 3218 cm-1 is assigned to the frequency of NH2 symmetric stretching vibration. The intense peak at 1727 cm-1 establishes the presence of C = O stretching vibration. The NH2 in - plane deformation vibration mode appears at 1644 cm-1. The C – N – H and C = C stretching vibrations are observed at 1497 cm-1 and 1368 cm-1 respectively. The peak at 1237 cm-1 occurs due to C – N stretching vibration. The strong band observed at 821 cm-1, 631 cm-1 and 428 cm-1 are due to the Se - O stretching vibration [15]. The observed wave numbers and the assignments are presented in Table 1.

Fig.3. FT – IR spectrum of CHS

Table 1. Tentative band assignment of FT - IR spectra for CSA single crystals

Wave number (cm-1)

Assignments

3316

NH2Asymmetric stretching vibration

3218

NH2 symmetric stretching vibration

1727

C = O stretching vibration

1644

NH2 deformation vibration

1497

C – N – H Stretching vibration

1368

C = C Stretching vibration

1237

C – N Stretching vibration

821

Se - O stretching vibration

631

Se - O stretching vibration

428

Se - O stretching vibration

3.3. UV - Vis - NIR Spectral analysis

The UV – Vis – NIR spectrum gives information about the changes in electronic structure of the molecule because the absorption of UV and visible light involves promotion of the electrons from the ground state to higher energy states. The UV – Vis – NIR transmittance and absorbance spectrum was recorded in the wavelength range of 190 – 1100 nm using Varian Cary 5E spectrophotometer. The UV transmittance and absorbance spectrum recorded for CHS of thickness 2mm is shown in Fig. 4. As there is no absorption in the entire UV – Vis – NIR, it can be used as potential material for frequency doubling process. The lower cutoff wavelength is observed at 290 nm. From the transmittance spectra optical parameters like absorption coefficient α and band gap were evaluated. The absorption coefficient (α) was evaluated from the equation α = 2.303 A/t, where t is the thickness and A is the absorbance of the crystal. The direct band gap was determined by fitting the absorption data to the equation αhѵ = B (αhѵ - hѵ)1/2 in which hѵ is the photon energy and B is the constant related to material. A plot of variation of hυ versus (αhυ)2 was drawn in Fig. 5 and the optical band gap (Eg) was obtained by extrapolating the linear part of the graph to X – axis. This gives a band gap value of 5.1 eV for CHS crystal.

Fig.4. UV – Vis – NIR transmittance and absorbance spectrum of CHS single crystal

Fig.5. Plot of (αhν)2 versus hν for CHS single crystal

3.4. Dielectric studies

The dielectric constant of a material gives information about the nature of atoms, ions and their bonding in the material. The dielectric constant and dielectric loss of the CHS crystals were studied at three different temperatures using a HIOKI 3532 LCR HITESTER instrument in the frequency range 50 Hz - 2 MHz. Cut and polished crystal of dimension 1cm x 1cm x 2mm was used for dielectric study. A two terminal copper electrode was used as a sample holder and the sample was held between the electrodes. The temperature of the sample was controlled and measured using a thermocouple. The thermocouple was fixed in the vicinity of lower electrode to measure the temperature of the sample. In this way a parallel plate capacitor was formed. The capacitance of the sample was measured by varying the frequency. The dielectric constant (ɛʹ) in the frequency range 50 Hz - 2 MHz was estimated at the temperature 32, 50 and 75 °C using the formula ɛʹ = Cd/(É›0A), where C is the capacitance of the crystal, d is the thickness of the crystal, A is the cross sectional area of the crystal and É›0 is the constant of permittivity of free space. The variation of the dielectric constant with log frequency at different temperature is shown in Fig. 6. It is found that dielectric constant has high values in the lower frequency region and then it decreases with increase in frequency. The dielectric constant of a material is composed of four contributions namely electronic, ionic, orientation and space charge polarizations. The high value of dielectric constant at low frequencies may be due to the presence of all the four polarizations and its low value at high frequencies may be due to the loss of significance of these polarizations gradually [16, 17]. It was observed from the graph that the dielectric constant of CHS exhibits a normal dielectric behaviour. Fig. 7 shows the exponential decrease of dielectric loss of CHS as a function of frequency.

Fig.6. Variation of dielectric constant with log frequency at various temperatures

Fig . 7. Variation of dielectric loss with log frequency at various temperatures

3.5Microhardness study

Microhardness measurement is a general microprose technique for assessing the bond strength, apart from being a measure of bulk strength. The hardness value correlated with other mechanical properties like elastic constants, yield strength, brittleness index and temperature of cracking. Microhardness measurements were carried at room temperature using Shimadzu HMV-2000 hardness tester fitted with a Vickers pyramid diamond. The load P is varied between 25g to 100g, and the indentation time is kept constant at 10s for all trails. The diagonal lengths of indentation were measured. The hardness of the material Hv is determined by the following relation.

Hv = 1.8544 P /d2 (Kg/mm2)

Fig.8 Microhardness values vs. load for CHS crystal

Where P is the applied load in Kg and d is the diagonal length of the impression in mm. The variation of hardness value and applied load is shown in Fig.8. The graph was plotted for log P versus log d is shown in Fig.9. The plot of log P versus log d yields a straight line and its slope gives the work hardening coefficient n. The value of n is found to be 5 for CHS crystal. Since the value of n is greater than 2, the hardness of the material is found increase with the increase of load. It confirms the prediction of Onitsch and also the reverse indentation size effect (RISE) [18 - 19].

Fig.9. log P vs. log d for CHS crystal

3.5. Thermal analysis

The thermo gravimetric ( TG ), differential thermal ( DT ) and differential scanning calorimetric (DSC) analysis were carried out using SDT Q600 v20.9 Build 20 for CHS sample weight of 7.9970 mg in the temperature range 25 to 500 °C at a heating rate of 10° C / min in nitrogen gas atmosphere. A small weight loss of 17.2 % observed in the range of 25° C - 150° C, which is assigned to the loss of selenous acid. There is a major weight loss of 69.5 % in the temperature range 150° C - 460° C. The second step of weight loss is attributed to the decomposition of cytosine molecules. The DTA trace illustrates two endothermic peak each at, 66.55° C and 167.47° C. The endotherms coincide with the weight losses shown in Fig.10

Fig. 10 TG/DTA and DSC trace of CHS single crystal

3.7. Etching studies

The chemical etching studies were carried out on the grown CHS crystal using polarized high resolution optical microscope fitted with Motic camera. Etching is an important tool for the identification of the crystal defects, such as growth hillocks, etch pits, grain boundaries on the crystal surface and micro structural imperfections of the grown crystal. Double distilled water was used as etchant. The photographs of the etch patterns are shown in Fig. 11a and Fig. 11b. When the etch time is 5s, which is shows less etch pits formed in the grown crystal surface. Etching study is made on the grown CHS single crystal with different etching time and when the etching time is increased, there is a major change observed in the morphology of the etch pits (with 10s).

Fig. 9a. Etching study on CHS crystal (etch time of 5s)

Fig. 9b Etching study on CHS crystal (etch time of 10s)

3.8. Second harmonic generation efficiency

The second harmonic generation (SHG) efficiency of the grown crystal was measured by using the Kurtz powder technique [20]. The fundamental beam of 1064 nm from Q-switched Nd:YAG laser ( Prolab 170 Quanta ray, pulse width 8 ns, repetition rate 10 Hz) was made to fall normally on the CHS crystalline powder densely packed in a capillary tube. The fundamental beam was filtered using an IR filter and the green radiation of 532 nm was collected by Photo multiplier tube (PMT-Philips photonics – model 8563). The optical signal incident on the PMT was converted into voltage output at the CRO (Tektronix – TDS 3052B). The input laser energy incident on the powdered sample was chosen to be 6.1mJ/pulse. A pure potassium dihydrogen Phosphate powdered sample of the same size of CHS (KDP) was used as the reference material and the result obtained for CHS shows a second harmonic generation efficiency of about 1.5 times that of KDP.

4. Conclusion

Slow evaporation technique at room temperature yielded CHS single crystals of 4 × 2 × 2 mm3 . The single crystal X – ray diffraction analysis reveals that the crystal belongs to orthorhombic system with a non centrosymmetric space group of PCa21. The crystallinity of the grown crystal was verified by powder X – ray diffraction analysis. Presence of various functional groups of CHS crystals was identified by FT – IR spectral studies. From the UV – Vis – NIR transmittance spectrum we found that the material has no absorption in the range of 210 - 1100 nm, thus confirming the suitability of CHS crystal for SHG application. The dielectric study reveals that the dielectric constant and dielectric loss decreases with increasing frequency at different temperatures. TG/DTA reveals that the compound is stable at room temperature and decomposes on increasing the temperature. Etching study is made on the surface of the grown crystal with different etching time and when the etching time is increased, there is no change in the morphology of the etch pits. The second harmonic generation efficiency of the crystal was measured by Kurtz powder technique and is 1.5 times that of KDP.

References

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